Should You Search Preprint Servers?

Should Preprints Be Part of Your Literature Search?

You search the major databases for your topic and find a solid body of published research. Then Google Scholar, a citation trail, or another search tool leads you to a manuscript on arXiv, bioRxiv, medRxiv, or another preprint server. It looks relevant and may even be more recent than anything you found in the journals.

Should it be part of your literature review?

Potentially, yes. Preprints can provide early access to research before the sometimes lengthy journal publication process is complete. They can be especially valuable in rapidly developing fields and for research questions where recent evidence matters. But they require careful handling because a preprint is not equivalent to a peer-reviewed journal article, and the version you find today may later be revised, published, or withdrawn.

Search Preprints When Early or Unpublished Research Matters

What Preprints Can Add to a Literature Search

What Is a Preprint?

A preprint is a scholarly manuscript made publicly available before, or sometimes while, it undergoes formal journal peer review. The Committee on Publication Ethics describes preprints as scholarly manuscripts posted by authors on openly accessible platforms, usually before or in parallel with peer review.

Preprints allow research findings to circulate without waiting for the complete journal editorial process. Depending on the field and server, manuscripts may receive basic screening before posting, but this should not be confused with the formal peer review conducted by a journal.

The distinction concerns publication status and review history. It does not tell you automatically whether the research itself is methodologically strong or weak.

Why Do Researchers Post Preprints?

Speed is one of the principal reasons.

Journal peer review, revision, acceptance, production, and publication take time. A preprint allows researchers to make a manuscript publicly available much earlier. COPE also identifies other potential benefits, including establishing precedence for a finding, obtaining feedback from a wider research community, facilitating interactions and collaboration, and providing evidence of research productivity.

Preprints can therefore function as part of an ongoing scholarly conversation rather than merely as unfinished substitutes for journal articles.

Their role varies considerably among disciplines. Preprint sharing has a long history in physics and mathematics, with arXiv operating since the early 1990s. Preprints became prominent later in many biomedical fields, particularly through platforms such as bioRxiv and medRxiv. Other disciplines have developed their own servers and communities.

Preprint Servers Are Not All the Same

A preprint server is a platform that hosts and disseminates preprints. Some are highly disciplinary, while others span several fields or support community-specific services.

This list is illustrative rather than exhaustive. Platforms change, new services emerge, and disciplinary practices evolve. Before searching a server, check its current scope, policies, screening practices, search functionality, and versioning procedures.

Preprints Can Reduce the Time Lag in Your Evidence Search

Imagine that an important study has been completed and written but is still moving through peer review. A conventional bibliographic search limited to formally published journal literature may not reveal it yet.

A preprint search might.

This can be especially valuable for fast-moving topics. During the COVID-19 pandemic, for example, preprints became an important channel for rapidly communicating biomedical and public-health research before conventional publication could keep pace.

The same principle can apply, at a less dramatic scale, to rapidly developing areas such as generative artificial intelligence, emerging technologies, computational methods, or newly developing scientific questions.

If your review needs to represent the research frontier rather than only the established journal record, the publication delay between manuscript completion and formal publication becomes relevant.

Searching Preprints Can Help Address Publication and Availability Gaps

Not every research manuscript becomes a conventional journal article quickly, and some may never do so. Preprints can therefore expose research that is not yet represented in journal databases.

This can make them relevant to searches concerned with grey literature or broader attempts to identify evidence beyond formally published journal articles.

However, the presence of a manuscript on a preprint server does not prove that it would satisfy the methodological or quality criteria of your review. Discovery and inclusion remain separate decisions.

You can therefore search preprint servers without committing yourself to include every preprint you find.

The Major Limitation Is That Preprints Have Not Usually Completed Formal Peer Review

This point should neither be minimized nor exaggerated.

Preprints can contain excellent research. Peer-reviewed articles can contain methodological weaknesses. Peer review is not a guarantee of correctness.

Nevertheless, a preprint has generally not yet passed through the formal editorial and peer-review process associated with journal publication. Reviewers may later identify analytical errors, unclear reporting, unsupported interpretations, or other problems. Authors may substantially revise the manuscript before journal publication.

The International Committee of Medical Journal Editors therefore recommends that preprint archives clearly identify preprints as work that has not been peer reviewed. In clinical and public-health contexts, where premature interpretation can have consequences beyond academia, this distinction is particularly important.

Preprints Can Change After You Find Them

One unusual feature of preprints is versioning.

Authors may upload revised versions after receiving comments, correcting errors, adding analyses, or making other changes. The manuscript may subsequently appear as a peer-reviewed journal article, and the published version may differ from the preprint.

Research on the use of preprints in evidence synthesis has highlighted the practical difficulty of matching preprints with later peer-reviewed publications and dealing with changes between versions.

When a preprint matters to your review, check its version history and search for a subsequent journal publication before finalizing your analysis. If a peer-reviewed version exists, compare it with the preprint rather than assuming that the title, sample, results, and conclusions remained unchanged.

Do Not Count a Preprint and Its Journal Article as Two Separate Studies

The same underlying study can appear first as a preprint and later as a journal article. Both records may therefore appear in your searches.

If you treat them as separate studies, you risk double counting the same participants, data, or findings.

This distinction is especially important in systematic reviews and meta-analyses. Study identification should occur at the study level, not merely at the document level.

A Preprint Later Becoming a Journal Article Does Not Make the Earlier Version Disappear

Preprint servers generally preserve the scholarly record rather than silently replacing the original manuscript with the journal article.

Researchers encountering an older preprint should therefore check whether a later version or published article is linked or can be identified through the title, authors, DOI, citation databases, or web searching.

For a literature review, the final peer-reviewed version will often be preferable when it represents the same study and is available. But the preprint remains relevant to understanding the publication history and may sometimes contain details altered or omitted later.

Preprint Searching Matters More in Some Disciplines Than Others

Preprint culture is not uniform across academia.

In fields where researchers routinely post manuscripts before journal publication, omitting preprint servers can leave a noticeable gap in very recent literature. In other disciplines, preprint use remains relatively uncommon, so a dedicated search may add little.

This means the decision should be based partly on how your research community communicates.

Look at recent papers in your field. Do authors cite arXiv, bioRxiv, medRxiv, PsyArXiv, or another server? Do major conferences or laboratories routinely circulate preprints? Are recent systematic reviews searching them? Does the field move quickly enough that several months of publication delay could materially change the evidence available?

These questions are more useful than a universal instruction to search or ignore preprints.

Preprints Are Particularly Relevant for Rapidly Emerging Topics

The faster a field changes, the greater the potential difference between the research that has been completed and the research that has completed journal publication.

A review of an established topic with decades of mature literature may gain relatively little from preprints. A review of a technology introduced eighteen months ago may encounter a very different publication landscape.

Preprints can therefore help reduce what might be called the publication-lag blind spot: the interval during which relevant research exists but has not yet entered the conventional journal record.

The trade-off is that the newest evidence may also be the least stabilized. Recency and evidential maturity are not the same thing.

Should Systematic Reviews Search Preprints?

There is no universal answer for every systematic review.

The decision depends on the research question, review protocol, eligible publication status, discipline, applicable methodological guidance, and likelihood that preprints contain relevant studies.

Preprints are commonly treated as a form of grey literature in evidence-synthesis searching. Recent methodological research comparing preprint search aggregators notes that grey-literature identification is often required or recommended in evidence-synthesis guidance and that preprint sources differ substantially in coverage and search functionality.

If preprints are eligible, the protocol should specify this rather than making the decision opportunistically after an interesting preprint appears. Review teams should also decide how they will identify later peer-reviewed versions, handle multiple versions, and report preprint status.

Research based on rapid reviews during COVID-19 similarly recommended having explicit procedures for identifying preprints, marking them clearly in reviews, checking peer-review status, and considering how their inclusion affects synthesis and conclusions.

Searching One Preprint Server May Not Search the Preprint Literature

Just as no conventional bibliographic database should automatically be assumed to contain everything, no single preprint server represents every discipline.

A psychology server and a biomedical server serve different research communities. An interdisciplinary question may require several servers or an aggregator that indexes multiple preprint sources.

Recent research comparing preprint aggregators found differences in server coverage, query functionality, advanced search operators, export capabilities, and version handling. This makes source selection important when preprints form a formal part of an evidence search.

If you are conducting a comprehensive review, investigate what each search service actually indexes rather than assuming that a general search interface covers every relevant preprint platform.

You May Already Be Finding Preprints Without Searching a Server Directly

Preprints can appear in broader scholarly search systems and databases. Google Scholar may retrieve them, and other research indexes or aggregators incorporate records from preprint platforms.

This can make direct server searching unnecessary for some exploratory tasks.

For a systematic search, however, incidental discovery is different from deliberate coverage. If preprints are an eligible and important source of evidence, document how you searched for them and which servers, databases, or aggregators were used.

Check Journal Policies If You Are Thinking About Posting Your Own Preprint

Searching preprints and posting your own manuscript are different decisions, but researchers often encounter both questions together.

Many journals accept manuscripts that have previously appeared as preprints, but policies vary. The ICMJE advises authors to become familiar with the policies of journals they intend to submit to before posting a preprint because prior posting can influence a journal's interest or publication decisions.

Do not infer a target journal's policy from general disciplinary practice. Verify the current instructions for authors or publisher policy directly.

When Searching Preprints Changes What You Know About an Emerging Topic

Searching preprints has extended the review closer to the research frontier. It has not transformed preliminary evidence into established evidence. That distinction is the methodological price of gaining speed.

Common Misunderstandings About Preprints

When Should You Add Preprint Servers to Your Search?

The decision becomes easier when you separate three questions: whether preprints are common in your field, whether recent unpublished research matters to your question, and whether your review methodology permits their inclusion.

Preprint searching should also fit the rest of your database strategy. If the topic crosses disciplines, determine whether subject-specific and multidisciplinary databases provide complementary coverage before assuming that preprints are the main source of missing evidence.

If your concern is unpublished academic work more broadly, preprints are only one category. Depending on the question, theses and dissertations, conference proceedings, reports, trial registers, or other grey-literature sources may matter as much or more.

Before Adding Preprints to Your Literature Review

Frequently Asked Questions About Preprint Servers

Preprints Can Bring Your Search Closer to the Research Frontier

How to Cite This Guide

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